Soft tissue paper is generally preferred for disposable paper towels, and facial and toilet tissues. However, known methods and means for enhancing softness of tissue paper generally adversely affect tensile strength. Tissue paper product design is, therefore, generally, an exercise in balancing softness against tensile strength.
Both mechanical and chemical means have been introduced in the pursuit of making soft tissue paper: tissue paper which is perceived by users, through their tactile sense, to be soft. A well known mechanical method of increasing tensile strength of paper made from cellulosic pulp is by mechanically refining the pulp prior to papermaking. In general, greater refining results in greater tensile strength. However, consistent with the foregoing discussion of tissue tensile strength and softness, increased mechanical refining of cellulosic pulp negatively impacts tissue paper softness, all other aspects of the papermaking furnish and process being unchanged.
A variety of chemical treatments have been proposed to increase the tactile sense of softness of tissue paper sheets. For example, it was disclosed in German Patent No. 3,420,940, Kenji Hara et al, to dip, impregnate, or spray dry tissue paper with a combination of a vegetable, animal, or synthetic hydrocarbon oil and a silicone oil such as dimethylsilicone oil. Among other benefits, the silicone oil is said to impart a silky, soft feeling to the tissue paper. This tissue paper, contemplated for toilet paper applications, suffers from disposal complications when flushed through pipe and sewer systems in that the oils are hydrophobic and will cause the tissue paper to float, especially with the passage of time subsequent to treatment with the oils. Another disadvantage is high cost associated with the apparent high levels of the oils contemplated.
It has also been disclosed to treat tissue paper and the furnish used to make tissue paper with certain chemical debonding agents. For example, U.S. Pat. No.3,844,880, Meisel Jr. et al, issued Oct. 29, 1974, teaches that the addition of a chemical debonding agent to the furnish prior to sheet formation leads to a softer sheet of tissue paper. The chemical debonding agents used in the Meisel Jr. et al process are preferably cationic. Other references, e.g., U.S. Pat. No. 4,158,594, Becker et al, issued Jan. 19, 1979 and Armak Company, of Chicago, Ill., in their bulletin 76-17 (1977) have proposed the application of cationic debonders subsequent to sheet formation. Unfortunately, cationic debonders in general have certain disadvantages associated with their use in tissue paper softening applications. In particular, some low molecular weight cationic debonders may cause excessive irritation upon contact with human skin. Higher molecular weight cationic debonders may be more difficult to apply in low levels to tissue paper, and also tend to have undesirable hydrophobic effects upon the tissue paper. Additionally, the cationic debonder treatments of these references tend to decrease tensile strength to such an extent that the use of substantial levels of resins, latex, or other dry strength additives is required to provide commercially acceptable levels of tensile strength. Such dry strength additives add substantial raw materials cost to the tissue paper due to the relatively high level of additive required to provide sufficient dry strength. Furthermore, many dry strength additives have a deleterious affect on tissue softness.
It has now been discovered that treating wet tissue paper webs with a noncationic surfactant results in significant improvement in the tissue paper's tensile/softness relationship relative to traditional methods of increasing softness. That is, the noncationic surfactant treatment of the present invention greatly enhances tissue softness and any accompanying decrease in tensile strength can be offset by traditional methods of increasing tensile strength such as increased mechanical refining. It has further been discovered that the addition of an effective amount of a binder, such as starch, to the wet tissue web will at least partially offset any reduction in tensile strength and/or increase in linting propensity that results from the noncationic surfactant.
While the present invention relates to improving the softness of paper in general, it pertains in particular to improving the tactile perceivable softness of high bulk, creped tissue paper. Representative high bulk, creped tissue papers which are quite soft by contemporary standards, and which are susceptible to softness enhancement through the present invention are disclosed in the following U.S. Pat. No. 3,301,746, Sanford and Sisson, issued Jan. 31, 1967; U.S. Pat. No. 3,974,025, Ayers, issued Aug. 10, 1976; U.S. Pat. No. 3,994,771 Morgan Jr. et al, issued Nov. 30, 1976; U.S. Pat. No. 4,191,609, Trokhan, issued Mar. 4, 1980 and U.S. Pat. No. 4,637,859, Trokhan; issued Jan. 20, 1987. Each of these papers is characterized by a pattern of dense areas: areas more dense than their respective remainders, such dense areas resulting from being compacted during papermaking as by the crossover knuckles of imprinting carrier fabrics. Other high bulk, soft tissue papers are disclosed in U.S. Pat. No. 4,300,981, Carstens, issued Nov. 17, 1981; and U.S. Pat. No. 4,440,597, Wells et al, issued Apr. 3, 1984. Additionally, achieving high bulk tissue paper through the avoidance of overall compaction prior to final drying is disclosed in U.S. Pat. No. 3,821,068, Shaw, issued Jun. 28, 1974; and avoidance of overall compaction in combination with the use of debonders and elastomeric bonders in the papermaking furnish is disclosed in U.S. Pat. No. 3,812,000, Salvucci Jr., issued May 21, 1974.
It is an object of this invention to provide a process for preparing tissue paper which has an enhanced tactile sense of softness.
It is a further object of this invention to provide a process for preparing tissue paper which has increased tactile softness at a particular level of tensile strength relative to tissue paper which has been softened by conventional techniques.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.